This invention relates to a servovalve, in particular for hydraulic power-assisted steering systems of motor vehicles, in the form of a rotary spool valve with a rotary spool and a control bush, which coaxially surrounds the rotary spool and can be rotated against spring force relative to the rotary spool.
Such servovalves are, for example, known from DE 42 34 571 A1 and U.S. Pat. No. 4,966,192. In the case of a power-assisted steering system, the rotary spool and the control bush form parts of a train of shafts which mechanically connects a manual steering wheel to the steered vehicle wheels. In this arrangement, a shaft part, which is torsionally connected to the control bush, and a shaft part, which supports or includes the rotary spool, are connected together by a torsion bar so that they are axially secured but torsionally flexible. The rotary spool and the control bush are deflected relative to one another from a relative central position to a greater or lesser extent corresponding to the forces and torques effective between the manual steering wheel and the vehicle steering wheels. This rotary travel between the control bush and the rotary spool has the effect that a hydraulic servomotor controlled by the servovalve is acted upon in a controllable manner by hydraulic pressure or by a hydraulic pressure difference in one or the other direction, depending on the direction of the relative twist between the control bush and the rotary spool supports the respective steering manoeuvre, i.e. the manual force to be applied to the manual steering wheel is correspondingly reduced.
The spring force is typically generated by a torsion bar which mechanically connects the rotary spool to the control bush. In order to accommodate the torsion bar, the rotary spool and the parts of the control bush axially abutting it or the elements torsionally connected to the control bush have a tubular configuration.
In DE 196 15 543 A, published after the claimed priority date, a servovalve, of the type mentioned at the beginning, is described in which a first axial extension is arranged eccentrically to the axis of the rotary spool valve at one end face of the control bush.
An end surface, which faces towards the aforesaid end face, has a further extension of the same type as the first extension and has the same radial distance from the axis of the rotary spool valve as the first extension. The end face is arranged at an axial distance from the free end of this extension on a part which is torsionally connected to the rotary spool, and the total axial length of the extensions is somewhat shorter than the axial distance between the end face of the control bush and the end surface.
A C-spring, which surrounds the rotary spool and whose free ends are clamped against the extensions and try to force the latter into a mutually aligned position, is arranged between the said end face of the control bush and the said end surface.
This realizes the general idea of arranging, axially between an end face of the control bush and a part at the rotary spool end, a ring-type spring, which surrounds a section of the rotary spool and which is bent open to a greater or lesser extent during relative rotations between the control bush and the rotary spool because the extensions then move away from one another in the peripheral direction of the rotary spool section.
A particular advantage of this design is the ease of assembly of the C-spring. In addition, the C-spring can also be configured with preload in such a way that a specified spring loading is still present when the pins are mutually aligned.
In accordance with this older patent application, provision is usually made for the rotary spool and the control bush to be additionally connected by way of a torsion bar, which is accommodated within a tubular inner space of the rotary spool and is firmly connected at one end to the rotary spool, for example by pinning, and is firmly connected at its other end to the control bush or parts connected to the latter. This torsion bar, as well as the aforesaid C-spring, effects an increasing restoring force in the case of relative rotations between the control bush and the rotary spool.
In addition, the torsion bar is used to secure the control bush and the rotary spool axially relative to one another.